In various environments, the dimensional and structural integrity of certain critical components of a particular system or apparatus is of the utmost importance in assuring against future failures. One example of such a critical component is the "cladding" tube of a nuclear fuel rod. Conventionally, a column of nuclear fuel pellets are sealed into a metal cladding tube, which is assembled along with other fuel containing cladding tubes in a lattice structure constituting the fuel rod assembly of a nuclear reactor. The purpose of "cladding" the nuclear fuel is to prevent chemical reactions between the nuclear fuel and the coolant or moderator, and also to prevent partially gaseous radioactive fission products from leaking into the coolant or moderator from the fuel. Moreover, in order to withstand various structural stresses experienced during their service life as nuclear fuel rods, cladding tubes must be manufactured to exacting standards of dimensional and structural integrity. Accordingly, cladding tube wall thickness is an important dimensional criteria since a thin wall section, which is less than a minimum tolerance dimension, may seriously jeopardize the ability of the fuel rod to withstand stresses imposed by differences in thermal expansion between the cladding and the contained fuel pellets. Likewise, inner and outer tube diameters must be precisely controlled such that fuel pellets can be properly loaded within the tube and so that the tubes can be properly assembled into fuel rod bundles.
The structural and compositional integrity of cladding tubes is also very important. Conventionally, a zirconium alloy (e.g., Zircaloy-2 or -4) is used as a preferred material in the construction of cladding tubes as it is well known for its neutron absorption characteristics, corrosion resistance, and chemical stability at elevated steady state temperatures. Moreover, in order to further reduce the possibility of incurring internal tube wall stress corrosion and/or cracking due to localized thermal expansion differences between the nuclear fuel and the cladding tube, a liner of substantially pure zirconium is conventionally bonded to the inner surface of a cladding tube. A pure zirconium liner is utilized because of its superior ability to moderate localized stress/strains occurring along the inner surface of a cladding tube during abrupt temperature changes. It is critically important that the zirconium liner be of a uniform predetermined thickness (typically on the order of 80-100 microns in thickness) to be effective.
Concerning the critical nature of nuclear fuel rod cladding tubes, it is essential to carefully and accurately inspect each cladding tube over its entire length for both dimensional and structural integrity before it can be utilized as a nuclear fuel rod. Prior to the present invention, the thickness of a cladding tube inner lining (also called a "barrier" layer) was determined by destructive metallography of representative samples. However, for obvious practical and economic reasons, it is highly desirable to ascertain compliance of the liner thickness with design specifications using non-destructive testing methods.
As can be appreciated by those skilled in the art, ultrasonic inspection methods utilizing a transducer in a pulse-echo mode are commonly known to provide non-destructive dimensional measurements of critical components for quality assurance. A transducer is scanned over the component, either by motion of the transducer and/or the component while the transducer is periodically electrically excited to admit a probing ultrasonic energy pulse. In the intervals between pulses, the transducer receives echoes which may be analyzed for dimensional information. Unfortunately, when used to inspect cladding tube structure, ultrasonic wave analysis cannot accurately distinguish liner or barrier layer boundary echoes from cladding tube inner surface echoes (presumably due to the similarity of the materials involved and the extreme thinness of the lining).
Non-destructive electromagnetic profilometry techniques are also well known in the art. Some exemplary eddy-current impedance measurement systems of the type are discussed in U.S. Pat. No. 4,741,878, U.S. Pat. No. 4,673,877 and U.S. Pat. No. 3,967,382. However, conventional electromagnetic testing systems that are used to detect various defects and changes in tube wall thickness are known to employ electro-mechanical probes that are inserted into the cladding tubes and generate eddy-currents. These conventional eddy-current probe systems utilize scanning arrangements that require maintaining the probe element in constant sliding contact with the inner surface of the tube throughout the duration of the test. Such an arrangement inevitably increases the chances of scratching or marring the cladding liner during the testing procedure and, thus, damaging the tube in the process. Moreover, eddy-current probe arrangements of this type are typically mechanically complex, costly to manufacture, difficult to use, and susceptible to inherent inaccuracies caused by "lift-off" variations (i.e., varying gaps or distances between the surface of the eddy-current probe measuring coils and the inner surface of the tube).
In accordance with an exemplary embodiment of the present invention, an improved method and apparatus is provided for determining the average circumferential thickness of a zirconium liner or "barrier" layer provided on the inside surface of a nuclear fuel rod cladding tube. In particular, the present invention utilizes conventional ultrasonic measurement techniques and conventional electromagnetic eddy-current analysis techniques in a combined system arrangement that involves a specific method for computing liner thickness which overcomes the aforementioned problems associated with prior art approaches. Briefly, a particular eddy-current probe arrangement consisting of a differential coil pair is employed to obtain impedance measurements from the outside of the cladding tube. Calibrated reference impedance values for different inner and outer cladding tube diameters with the same liner thickness are measured from a calibration standard and retained in a memory. Calibrated reference impedance values for various liner thicknesses with the same inside and outside diameter are also measured and stored. A specimen cladding tube is then tested and an inside diameter is computed using conventional electromagnetic techniques. The specimen tube inside diameter is then measured ultrasonically and a specific calculation of liner thickness is performed based on the difference between the inside diameter as determined by the ultrasonic technique and the inside diameter as measured by the eddy-current technique. A specific computation is then performed which corrects for the erroneous effect that variations in cladding tube liner thickness can have on dimensions computed via conventional electromagnetic eddy-current techniques alone.
Eddy-current impedance measurements of a fuel rod cladding tube, however, are greatly effected by the respective compositions of the cladding tube and liner (e.g., the overall conductance of an induced eddy-current is dependent on the relative proportions of Zircaloy-2 and zirconium in the cladding). Consequently, the tube under test must necessarily have the same liner thickness (or relative proportions of respective metals) as whatever standard is used to initially calibrate the eddy-current measurement apparatus otherwise the measured conductances will not accurately reflect actual inner and outer diameter dimensions. Since this is obviously not feasible, inside diameter measurements as determined by eddy-current electromagnetic techniques may not correspond exactly to the actual inside diameter. Consequently, in accordance with a preferred embodiment of the present invention, cladding tube inside diameter is determined by conventional ultrasonic techniques as well as electromagnetic techniques. The difference between the "actual" inside diameter as determined by ultrasonic techniques and the inside diameter as measured by the above eddy-current technique is then used to accurately calculate the actual thickness of a cladding tube liner by a computational method discussed herein in greater detail below.
Although the present invention was developed as a result of a need for improved techniques to accurately measure the interior liner thickness of nuclear fuel rod cladding tubes, it would be appreciated by those skilled in the art that the invention has other uses, such as, for example, determining the thickness of metallic linings or coatings on other tubular members or other types of conduits.